I'm a geochemist. In the past ten years I've fixed mass spectrometers, blasted sapphires with a laser beam, explored for uranium in a nature reserve, and measured growth patterns in fish ears, and helped design the next generation of the world's most advanced ion probe. My main interest is in-situ mass spectrometry, but I have a soft spot in my heart for thermodynamics, drillers, and cosmochemistry.

Wednesday, October 01, 2008

As I mentioned a while back, the presence of short-lived radioactive nuclides in early solar system material means that a nucleosynthetic event must have occurred shortly (< a few million years) before these materials formed. The simplest explanation for this is that a supernova occurred, in which these elements were produced. This supernova then injected these materials into a nearby molecular cloud, and the force of the shockwave triggered the collapse of this cloud to form our solar system. It all makes a good story, and the simplicity of the model makes interpretation of cosmochemical data more straightforward than a complicated model would. The problem is, a gang of unruly astrophysicists eventually found out about it and attacked it with calculations. Gounelle and Meibom were particularly savage.

Their paper is basically argued like this:1. injection and collapse from a single supernova is unlikely.2. contamination of a circumstellar disk in unlikely (this is the hypothesis they discuss in detail).3. Therefore, the excess short-lived nuclides must have been inherited from the stellar medium, or formed from spallation or other non-stellar processes.

The argument against single-supernova injection and collapse is fairly simple. Basically, the computer says "No".

Specifically, they argue that models show that fast supernova ejecta will disperse the cloud instead of collapsing it, while low speed ejecta will not penetrate the cloud, so injection does not occur.

They then turn to the supernova ‘salting’ of a circumstellar disk.The problem here is that prior to blowing up, stars that are big enough to end in supernovae are very luminous and hot, and thus emit a huge amount of UV. This UV will evaporate any disks nearby on timescales shorter than the life of the massive star. So by the time the star blows up, there is no disk left to inject materials into. They calculate that the probability of a disk surviving long enough, but still being close enough to receive sufficient material, is very small.

Having thus shown these two most popular theories to be unlikely, they conclude that spallation is the best hypothesis for 26Al and other short lived nuclides, while the longer lived ones (halflives > a few million years) live long enough to have been present in the interstellar medium before star formation.

I generally don’t like elimination-based arguments for poorly constrained problems like this one. In order for not 1 and not 2 to imply something, you have to also exclude the possibility of a door number 3. Additionally, they made the mirror image mistake of cosmochemists- despite their expertise in star formation, they don’t acknowledge that meteoritic concentrations of 10Be (which has to be spallation related) and 26Al are not correlated, at least in CAIs. This suggests that the origin of 26Al is not spallation.

But while I disagree with their argument, the bigger picture is still important- meteoriticists can’t just sit in a lab measuring obscure isotopic ratios without verifying the astronomical plausibility of our chemically-derived models. Like it or not, it is in our best interests to actually talk to these people, even if their skill sets, publishing habits, and scientific culture is different and strange. Because despite the vast differences in methodology between high-precision isotope geochemistry and whatever it is that astronomers actually do, we are in fact attacking the same problem: How do stellar systems form.

Matthieu Gounelle and Anders Meibom 2008 THE ORIGIN OF SHORT-LIVED RADIONUCLIDES AND THE ASTROPHYSICAL ENVIRONMENT OF SOLAR SYSTEM FORMATION The Astrophysical Journal, 680:781–792

Matthieu Gounelle, Anders Meibom (2008). THE ORIGIN OF SHORT-LIVED RADIONUCLIDES AND THE ASTROPHYSICAL ENVIRONMENT OF SOLAR SYSTEM FORMATION The Astrophysical Journal

4 comments:

I do find it quite interesting that there is not more communication between the various genres of scientific research. Glad to be able to crash someone's party, though. :-)When you say there's no correlation between 10Be and 26Al concentrations in meteoritic grains, what precisely do you mean? Does that assessment take into account the differing cross-sections for 10Be and 26Al production via spallation? Many of the experiments of which I've been a part are attempts to measure the rates involved in the large nuclear reaction networks which produce such isotopes (18F, 26Al, 10Be, 131Sn, etc); the fact that we haven't yet truly constrained many of these rates is, I would think, an important point to consider when trying to determine the possible sources for such nuclei.

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